Ok, dwanthny was kind enough to send me an example epub - thx very much! What I hypothesised was correct. The older style jackets had no <meta> tags.

I'm looking at the jacket.xhtml file and wondering what might distinguish this as a Calibre jacket versus one that the original epub author coincidentally called jacket.xhtml. I can see why Kovid added the <meta> tag... lol

I welcome any suggestions... below is the jacket (dwanthny I have removed any identifying content in case it was a problem).